1. Field of the Invention
The present invention relates to flat cables, more particularly to a flat cable for use in high-frequency signal transmission.
2. Description of the Related Art
As an invention related to a conventional flat cable, a high-frequency signal line described in, for example, International Publication No. WO2012/073591 (for example, see FIG. 9) is known. This high-frequency signal line includes a dielectric element assembly, a signal line, and two ground conductors. The dielectric element assembly is formed by laminating a plurality of dielectric sheets. The signal line is provided in the dielectric element assembly. The two ground conductors are provided in the dielectric element assembly such that the signal line is positioned therebetween in the direction of lamination. As a result, the signal line and the two ground conductors constitute a stripline structure.
Furthermore, each of the two ground conductors has a plurality of openings provided therein, and the openings overlap with the signal line when viewed in a plan view in the direction of lamination. This results in less capacitance being created between the signal line and the two ground conductors. Therefore, it is possible to reduce the distance between the signal line and the ground conductors in the direction of lamination, so that the high-frequency signal line can be reduced in thickness.
However, the high-frequency signal line described in International Publication No. WO2012/073591 has a problem in that the characteristic impedance of the signal line might fluctuate. More specifically, the high-frequency signal line described in International Publication No. WO2012/073591 is attached to a metallic object such as a battery pack. In this case, since the openings are provided in both of the ground conductors, the signal line faces the battery pack through the openings regardless of which side of the high-frequency signal line is directed to the battery pack. Accordingly, there is some capacitance created between the signal line and the battery pack, resulting in fluctuations in the characteristic impedance of the signal line.
Note that International Publication No. WO2011/007660 also describes a signal line having a stripline structure. This signal line also has openings provided in two ground conductors, and therefore, has a problem with fluctuations in the characteristic impedance of the signal line.